This invention relates to permanent magnet motors of the interior or embedded magnet type.
Permanent magnet motors have good operating characteristics at synchronous speed, but the induction cages used to start them require large starting currents and give highly variable starting torques. The high starting currents for the induction starts undesirably tend to demagnetize the permanent magnet material, a particular problem for ferrite magnets. Demagnetization renders the motor inoperative as a synchronous motor. Another problem is that since the induction torque is zero at synchronous speed, the permanent magnet rotor does not always pull into synchronism especially with a high inertia load. Failure to pull into synchronism causes overheating and the motor does not run as a synchronous motor.
It is an object of the present invention to provide a permanent magnet motor having uniform average or net starting torque.
It is a further object of the present invention to provide a permanent magnet motor with a starting current no larger than the pull-out current of the motor when it is operating in synchronism.
It is a still further object of the present invention to provide a permanent magnet motor that can pull into synchronism large inertia loads.